People living with HIV (PLHIV) who also have co-occurring substance abuse disorders experience a greater number of negative health outcomes than PLHIV without these disorders. In fact, research has repeatedly shown that substance use is one of the most consistent predictors of poor adherence to HIV care and antiretroviral therapy (ART) medications. Poor adherence to HIV care and ART, in turn, are associated with inadequate suppression of viral load and adverse health outcomes. Because substance abuse presents difficulties for HIV treatment adherence, and substance users have a higher likelihood of HIV transmission through risky sexual behaviors and injection drug use, this population has a pressing need for assistance with engagement in HIV care. While substance use is a known barrier to retention in HIV care and ART medication adherence, the underlying mechanisms that drive this relationship remain understudied. Health-related stigma is one significant barrier to HIV treatment. Stigma not only affects mental health and quality of life for PLHIV, but also may influence retention in HIV care and adherence to ART medications. An emerging body of literature has revealed that PLHIV who also have co-occurring substance use disorders may experience multiple, intersecting, or layered stigmas. Although the potential roles of these stigmas in adherence to HIV care have been discussed, understanding how HIV- and substance abuse (SA)-related stigmas may interact to influence health behaviors remains a significant gap in the literature. In-depth understanding of how PLWH with co-occurring substance use disorders experience these stigmas and how these stigmas affect adherence to treatment is needed in order to develop appropriate interventions. Utilizing a mixed-methods design, the specific aims of this study will fill several gaps in the literature by providing the first investiation of how SA-related stigma and the intersection of HIV- and SA-related stigmas influence ART adherence and retention in HIV care. Combining quantitative and qualitative methods will provide a more in-depth understanding of psychosocial barriers to engagement in HIV care than either method alone. Finally, this research will use data collected using validated stigma measures and clinical data from one of the seven original Centers established by NIAID to stimulate scientific advancement in HIV/AIDS research-UAB's Center for AIDS Research (CFAR). UAB's CFAR not only provides the ideal environment for the current research, but also provides the ideal environment for the training of young investigators. Training received throughout the fellowship period and the information gained about engagement in HIV care through the proposed research will inform subsequent research focusing on the development of stigma-reduction interventions to improve engagement in HIV care among PLWH with co-occurring substance use disorders.